Light in a mid-infrared wavelength region (for example, at a wavelength of 5 to 30 μm) has become an important wavelength region in the field of spectroscopic analysis. As a high-performance semiconductor light source in such a wavelength region, in recent years, quantum cascade lasers (QCL) have gained the spotlight (for the quantum cascade lasers, refer to Patent Documents 1 to 4 and Non Patent Documents 1 to 4, for example).
The quantum cascade laser is a monopolar type laser element that generates light by an electron transition between subbands by utilizing a level structure with subbands, which is formed in a semiconductor quantum well structure, that is capable of realizing high-efficiency and high-power operations by cascade coupling quantum well emission layers in multiple stages which are formed by quantum well structures to be an active region. Further, the cascade coupling of the quantum well emission layers is realized by using an electron injection layer for injecting electrons into the emission upper level, and alternately laminating the quantum well emission layers and the injection layers.    Patent Document 1: U.S. Pat. No. 5,457,709    Patent Document 2: U.S. Pat. No. 5,745,516    Patent Document 3: U.S. Pat. No. 6,751,244    Patent Document 4: U.S. Pat. No. 6,922,427    Non Patent Document 1: M. Beck et al., “Continuous Wave Operation of a Mid-Infrared Semiconductor Laser at Room Temperature,” Science Vol. 295 (2002) pp. 301-305    Non Patent Document 2: J. S. Yu et al., “High-Power Coutinuous-Wave Operation of a 6 μm Quantum-Cascade Laser at Room Temperature,” Appl. Phys. Lett. Vol. 83 (2003) pp. 2503-2505    Non Patent Document 3: A. Evans et al., “Continuous-Wave Operation of λ˜4.8 μm Quantum-Cascade Lasers at Room Temeprature,” Appl. Phys. Lett. Vol. 85 (2004) pp. 2166-2168    Non Patent Document 4: A. Tredicucci et al., “High Performance Interminiband Quantum Cascade Lasers with Graded Superlattices,” Appl. Phys. Lett. Vol. 73 (1998) pp. 2101-2103